Slim sxc/pages/__init__.py for federation, test, cart, blog

Move render functions, layouts, helpers, and utils from __init__.py
to sub-modules (renders.py, layouts.py, helpers.py, utils.py).
Update all bp route imports to point at sub-modules directly.
Each __init__.py is now ≤20 lines of setup + registration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

federation/bp/identity/routes.py

@@ -33,7 +33,7 @@ async def _render_choose_username(*, actor=None, error="", username=""):
     from shared.sx.helpers import render_to_sx
     from shared.sx.parser import SxExpr
     from shared.sx.page import get_template_context
-    from sxc.pages import _social_page
+    from sxc.pages.utils import _social_page
     from markupsafe import escape
 
     ctx = await get_template_context()